| 1 | --- |
| 2 | id: viewing-and-analyzing-your-traces |
| 3 | --- |
| 4 | |
| 5 | This section describes how to visualize the data gathered after tracing |
| 6 | the Linux kernel or a user space application. |
| 7 | |
| 8 | Many ways exist to read LTTng traces: |
| 9 | |
| 10 | * **`babeltrace`** is a command line utility which converts trace formats; |
| 11 | it supports the format used by LTTng, |
| 12 | <abbr title="Common Trace Format">CTF</abbr>, as well as a basic |
| 13 | text output which may be `grep`ed. The `babeltrace` command is |
| 14 | part of the |
| 15 | <a href="http://diamon.org/babeltrace" class="ext">Babeltrace</a> project. |
| 16 | * Babeltrace also includes **Python bindings** so that you may |
| 17 | easily open and read an LTTng trace with your own script, benefiting |
| 18 | from the power of Python. |
| 19 | * **<a href="http://tracecompass.org/" class="ext">Trace Compass</a>** |
| 20 | is an Eclipse plugin used to visualize and analyze various types of |
| 21 | traces, including LTTng's. It also comes as a standalone application. |
| 22 | |
| 23 | LTTng trace files are recorded in the `~/lttng-traces` directory by default. |
| 24 | Let's now view the trace and perform a basic analysis using |
| 25 | `babeltrace`. |
| 26 | |
| 27 | The simplest way to list all the recorded events of a trace is to pass its |
| 28 | path to `babeltrace` with no options: |
| 29 | |
| 30 | <pre class="term"> |
| 31 | babeltrace ~/lttng-traces/my-session |
| 32 | </pre> |
| 33 | |
| 34 | `babeltrace` finds all traces recursively within the given path and |
| 35 | prints all their events, merging them in order of time. |
| 36 | |
| 37 | Listing all the system calls of a Linux kernel trace with their arguments is |
| 38 | easy with `babeltrace` and `grep`: |
| 39 | |
| 40 | <pre class="term"> |
| 41 | babeltrace ~/lttng-traces/my-kernel-session | grep sys_ |
| 42 | </pre> |
| 43 | |
| 44 | Counting events is also straightforward: |
| 45 | |
| 46 | <pre class="term"> |
| 47 | babeltrace ~/lttng-traces/my-kernel-session | grep sys_read | wc --lines |
| 48 | </pre> |
| 49 | |
| 50 | The text output of `babeltrace` is useful for isolating events by simple |
| 51 | matching using `grep` and similar utilities. However, more elaborate filters |
| 52 | such as keeping only events with a field value falling within a specific range |
| 53 | are not trivial to write using a shell. Moreover, reductions and even the |
| 54 | most basic computations involving multiple events are virtually impossible |
| 55 | to implement. |
| 56 | |
| 57 | Fortunately, Babeltrace ships with Python 3 bindings which makes it |
| 58 | really easy to read the events of an LTTng trace sequentially and compute |
| 59 | the desired information. |
| 60 | |
| 61 | Here's a simple example using the Babeltrace Python bindings. The following |
| 62 | script accepts an LTTng Linux kernel trace path as its first argument and |
| 63 | prints the short names of the top 5 running processes on CPU 0 during the |
| 64 | whole trace: |
| 65 | |
| 66 | ~~~ python |
| 67 | import sys |
| 68 | from collections import Counter |
| 69 | import babeltrace |
| 70 | |
| 71 | |
| 72 | def top5proc(): |
| 73 | if len(sys.argv) != 2: |
| 74 | msg = 'Usage: python {} TRACEPATH'.format(sys.argv[0]) |
| 75 | raise ValueError(msg) |
| 76 | |
| 77 | # a trace collection holds one to many traces |
| 78 | col = babeltrace.TraceCollection() |
| 79 | |
| 80 | # add the trace provided by the user |
| 81 | # (LTTng traces always have the 'ctf' format) |
| 82 | if col.add_trace(sys.argv[1], 'ctf') is None: |
| 83 | raise RuntimeError('Cannot add trace') |
| 84 | |
| 85 | # this counter dict will hold execution times: |
| 86 | # |
| 87 | # task command name -> total execution time (ns) |
| 88 | exec_times = Counter() |
| 89 | |
| 90 | # this holds the last `sched_switch` timestamp |
| 91 | last_ts = None |
| 92 | |
| 93 | # iterate events |
| 94 | for event in col.events: |
| 95 | # keep only `sched_switch` events |
| 96 | if event.name != 'sched_switch': |
| 97 | continue |
| 98 | |
| 99 | # keep only events which happened on CPU 0 |
| 100 | if event['cpu_id'] != 0: |
| 101 | continue |
| 102 | |
| 103 | # event timestamp |
| 104 | cur_ts = event.timestamp |
| 105 | |
| 106 | if last_ts is None: |
| 107 | # we start here |
| 108 | last_ts = cur_ts |
| 109 | |
| 110 | # previous task command (short) name |
| 111 | prev_comm = event['prev_comm'] |
| 112 | |
| 113 | # initialize entry in our dict if not yet done |
| 114 | if prev_comm not in exec_times: |
| 115 | exec_times[prev_comm] = 0 |
| 116 | |
| 117 | # compute previous command execution time |
| 118 | diff = cur_ts - last_ts |
| 119 | |
| 120 | # update execution time of this command |
| 121 | exec_times[prev_comm] += diff |
| 122 | |
| 123 | # update last timestamp |
| 124 | last_ts = cur_ts |
| 125 | |
| 126 | # display top 10 |
| 127 | for name, ns in exec_times.most_common(5): |
| 128 | s = ns / 1000000000 |
| 129 | print('{:20}{} s'.format(name, s)) |
| 130 | |
| 131 | |
| 132 | if __name__ == '__main__': |
| 133 | top5proc() |
| 134 | ~~~ |
| 135 | |
| 136 | Save this script as `top5proc.py` and run it with Python 3, providing the |
| 137 | path to an LTTng Linux kernel trace as the first argument: |
| 138 | |
| 139 | <pre class="term"> |
| 140 | python3 top5proc.py ~/lttng-sessions/my-session-.../kernel |
| 141 | </pre> |
| 142 | |
| 143 | Make sure the path you provide is the directory containing actual trace |
| 144 | files (`channel0_0`, `metadata`, and the rest): the `babeltrace` utility |
| 145 | recurses directories, but the Python bindings do not. |
| 146 | |
| 147 | Here's an example of output: |
| 148 | |
| 149 | ~~~ text |
| 150 | swapper/0 48.607245889 s |
| 151 | chromium 7.192738188 s |
| 152 | pavucontrol 0.709894415 s |
| 153 | Compositor 0.660867933 s |
| 154 | Xorg.bin 0.616753786 s |
| 155 | ~~~ |
| 156 | |
| 157 | Note that `swapper/0` is the "idle" process of CPU 0 on Linux; since we |
| 158 | weren't using the CPU that much when tracing, its first position in the list |
| 159 | makes sense. |